Year	per 1,000
1990	24.5
1991	24.2
1992	24
1993	23.8
1994	23.6
1995	23.4
1996	23.2
1997	23
1998	22.8
1999	22.5
2000	22.2
2001	21.9
2002	21.6
2003	21.3
2004	20.9
2005	20.6
2006	20.2
2007	19.9
2008	19.5
2009	19.1
2010	18.8
2011	18.4
2012	18.1
2013	17.8
2014	17.5
2015	17.1
2016	16.8
2017	16.4
2018	16.1
2019	15.8
2020	15.5
2021	15.2
2022	14.9